This invention relates to the fields of NMR spectroscopy and imaging and, more particularly, to a method for correcting for variations in the phase and amplitude between two NMR signals acquired in successive experiments or scans.
The source of variation in a signal acquired during successive NMR experiments is principally the result of changes in the strength of the applied polarizing magnetic field B.sub.o. Changes in B.sub.o may result from physical changes in the NMR magnet, because of vibration or thermal stress, or may result from the influence of outside objects on the magnetic field.
Variations in the amplitude of the signal between successive NMR experiments occurs because of the additional step of averaging several signals acquired during each experiment into a composite signal for that experiment. Phase shifts between such multiple signals, acquired in a given experiment, translate to amplitude variations in the averaged signal.
The need for precisely reproducible spectroscopy signal phase and amplitude arises from a class of spectroscopy techniques which investigate small signals, such as those arising from proton coupling between certain elements, in the presence of larger unwanted signals, such as uncoupled proton resonances from water. Suppressing the larger, obscuring signals is performed by designing an experiment where successively acquired signals are identical but for a phase difference in the signal of interest. Subtracting these successive signals cancels the unwanted, obscuring signal which remains constant between the experiments. The signal of interest, whose phase was changed, is not cancelled. Frequently, the obscuring signal is several orders of magnitude greater than the signal of interest. Therefore it is essential that the phase and amplitude of the obscuring signals remain constant between successive experiments, otherwise, after the subtraction process a substantial component of the unwanted signal will remain uncancelled.
One such spectroscopy experiment, where phase and amplitude stability is critical, is described in co-pending U.S. patent application Ser. No. 07/181,996 which describes a number of techniques for cancelling unwanted NMR signals by adjusting the phase of only certain components of these signals between successive experiments.
In the area of NMR imaging, precise phase and amplitude stability may improve the signal-to-noise ratio of the resulting signal and the resolution of the image or spectra which is dependent, in one or more axis, on phase information. Imaging techniques such as "spin echo", because of their long acquisition delays, are particularly susceptible to phase error.